Pentose Phosphate Pathway Generation of NADPH and Pentoses

Overview Function NADPH production Reducing power carrier Synthetic pathways Role as cellular antioxidants Ribose synthesis Nucleic acids and nucleotides

Characteristics: Tissue Distribution Demand for NADPH Biosynthetic pathways FA synthesis (liver, adipose, mammary) Cholesterol synthesis (liver) Steroid hormone synthesis (adrenal, ovaries, testes) Detoxification (Cytochrome P-450 System) – liver Reduced glutathione as an antioxidant (RBC) Generation of superoxide (neutrophils)

Characteristics: Oxidative and Non-oxidative Phases Oxidative phases Reactions producing NADPH Irreversible Non-oxidative phases Produces ribose-5-P Reversible reactions feed to glycolysis

NADPH producing reactions Glucose-6-P dehydrogenase 6-P-gluconate dehydrogenase

The Pentose Phosphate Pathway: Non-oxidative phases

Regulation Glucose-6-P dehydrogenase First step Rate limiting Allosteric Regulation Feedback inhibited by NADPH Inducible enzyme Induced by insulin

Role of NADPH in the RBC Production of superoxide Hb-Fe 2+ -O 2 -> Hb-Fe 3+ + O 2 -. Spontaneous rxn, 1% per hour O H 2 O -> 2H 2 O 2 Both O 2 -. & H 2 O 2 can produce reactive free radical species, damage cell membranes, and cause hemolysis

Detoxification of Superoxide Anion and Hydrogen Peroxide Antioxidant enzymes Superoxide dismutase Glutathione peroxidase Glutathione reductase

Case Study 21 yo male medical student with malaria Treated with primaquine Four days later: Black colored urine Low RBC count Elevated reticulocyte count RBC with Heinz bodies Low hemoglobin Elevated serum bilirubin Pt recovered in a few days

G6PDH Deficiency and Hemolytic Anemia Most common genetic enzymopathy 400 hundred variants of G6PDH deficiency Mediterranean, Asian, African descent 400 million people affected worldwide 50% of Kurdish men 10-14% of African-American men with G6PD deficiency

Worldwide distribution of G6PD deficiency: 1995

G6PD Deficiency Distribution of G6PD deficiency coincides prevalence of malaria G6PD deficiency may impart some degree of malaria resistance Also sickle cell anemia

Genetics Recessive sex-linked mutation X-chromosome Rare in females (two X-chromosomes) Homozygous mutation: high hemolysis and anemia Heterozygous mutation: Normally asymptomatic unless exposed to drugs (primaquine, anti-malarial drug) or compounds (fava bean) that produce superoxide or hydrogen peroxide

Inheritance of G6PD Deficiency

G6PD Deficiency Exposure to anti-malarial drugs (Primaquine) results in increased cellular production of superoxide and hydrogen peroxide (Primaquine sensitivity) Other chemicals known to increase oxidant stress Sulfonamides (antibiotic) Asprin and NSAIDs Quinadine and quinine Napthlane (mothballs) Fava beans (vicine & isouramil)

Fava Beans Grown worldwide Important in Middle East High in protein Frost resistant perennial Genetically modified fava bean being developed Low in vicine and isouramil Favism

Case Study 21 yo male medical student with malaria Treated with primaquine Four days later: Black colored urine Low RBC count Elevated reticulocyte count RBC with Heinz bodies Low hemoglobin Elevated serum bilirubin Pt recovered in a few days

Symptoms Black colored urine Hemolysis may result in urinary excretion of hemoglobin Low RBC count & low hemoglobin Result of high rate of hemolysis Elevated bilirubin Catabolism of heme

RBCs with Heinz Bodies Precipitation of hemoglobin due to disulfide bond formation between Hb molecues Upper photo shows distorted RBCs with large Heinz bodies Bottom photo shows RBC stained with methylene blue

Elevated Reticulocytes A RBC containing granules or filaments representing an immature stage in cell development Normally constitutes 1% of circulating RBCs Reticulocytosis Elevation of reticulocytes Indicative of active erthropoiesis in red bone marrow

Defective G6PDH Results in enzyme with unstable structure Patient with 10% of normal activity Enough to generate NADPH under normal condition Newly made RBCs have normal 6PDH activity Patients recover quickly (8 days)